I think I have solved the issue with GH-615M HRM belt:
My belt has been totally unusable for two weeks as it has shown not only high spikes but almost all the time >200bpm HR.
I managed to "listen" to the HR belt transmitter by using a radio receiver very close to the belt (radio receiver IF or audio circuit was able to capture the 5kHz signal).
I could hear in the beginning the HR to be constant. Then, I made very minimal movements + pushed the belt just a couple of millimeters by hand. Immediately, when moving, the belt started to transmit spikes. Again, at the end the HR was constant as I didn't move.
The same happens when I kept the belt in my fingertips close to the radio and moved my hands or bent the belt just a few mms.
conclusion->the transmitter belt is broken (maybe some electrical/mechanical contact is broken) and this causes the belt to transmit disturbing signals.
...or this was my first thought...
then, I paid attention to the belt and guessed that the rubber parts must be connected to the center part by some kind of electromechanical joints - based on the experience with polar HRM belts, I thought that the joints, as they are buried inside of the structure, can capture all the dirt, sweat and soap, if used.
->I went to bathroom and rinsed the joints very long with warm water and then put the whole thing under water for some minutes.
->today it was the first time for loooong time when GH-615 showed real HR. (only 2-3 spikes, but they disappeared immediately when I pulled the belt away from chest for a second or two. (I think my belt needs still some water rinsing in order to work perfectly)
I hope this helps some of you...